The following BBW (brake by wire) type brake device is known by Patent Document 1 given below. In the BBW type brake device, an operation of a brake pedal by a driver is converted to an electric signal, and wheel cylinders are operated by a brake fluid pressure generated by a slave cylinder which operates according to the electric signal. A master cylinder that is operated by the brake pedal is provided as a backup for the case in which the slave cylinder fails to operate. While the slave cylinder is operating normally, the brake pedal is allowed to stroke by a stroke simulator absorbing brake fluid delivered by the master cylinder.    Patent Document 1: Japanese Patent Application Laid-open No. 2008-110633